In cellular communications systems, for example in a wireless cellular communications system using CDMA (code division multiple access) techniques for communications between a mobile terminal or station (MS) and a base station (BS), it is well known for the BS to transmit a pilot signal and a broadcast message including a preamble. On being powered up in a cell associated with the BS, a MS uses the pilot signal for synchronization to the BS, and downloads information including the preamble from the broadcast message. Having accordingly determined the timing of the BS, the MS transmits the preamble on the RACH. This is detected by the BS using correlation techniques, so that the BS is informed of the MS, and it can proceed with establishing communications on a traffic channel between the BS and the MS.
In submission TSGR1#3(99)205 to the TSG-RAN Working Group 1 meeting #3, Mar. 22-26, 1999, entitled “New RACH preambles with low auto-correlation sidelines and reduced detector complexity”, it is proposed that the preamble, which comprises 4096 code chips providing one of 16 orthogonal signatures of length 16 complex signals, be provided by binary Golay sequences, which have the advantageous property that the sum of their aperiodic auto-correlation functions is zero for all non-zero time shifts. Consequently, that submission purposes that the preamble be formed from a pair of complementary sequences A and B, which together constitute a Golay sequence and are referred to as constituent Golay sequences, each of 256 chips, repeated in a specific one of 16 signature patterns, so that the overall sequence has a length of 4096 chips, as shown by Table 1 below.
In Table 1, the signature patterns include the sequences A and B in normal and inverted forms, the inverted forms being denoted −A and −B respectively. The 4096 chips of an overall sequence can conveniently be included in one 10 ms time slot, the constituent Golay sequences A and B being individual to a specific cell and/or BS, and the above signature patterns being the same for all cells and base stations.
TABLE 11AABBA−A −B BA−A B−B AA−B −B 2AABBA−A −B B−A A−B B−A −A BB3A−A B−B AA−B −B AABBA−A −B B4A−A B−B AA−B −B −A −A −B −B −A AB−B 5AABB−A AB−B A−A B−B −A −A BB6AABB−A AB−B −A A−B BAA−B −B 7A−A B−B −A −A BBAABB−A AB−B 8A−A B−B −A −A BB−A −A −B −B A−A −B B9AA−B −B A−A B−B A−A −B BAABB10 AA−B −B A−A B−B −A AB−B −A −A −B −B 11 A−A −B BAABBAA−B −B A−A B−B 12 A−A −B BAABB−A −A BB−A A−B B13 AA−B −B −A A−B BA−A −B B−A −A −B −B 14 AA−B −B −A A−B B−A AB−B AABB15 A−A −B B−A −A −B −B AA−B −B −A A−B B16 A−A −B B−A −A −B −B −A −A BBA−A B−B 
In contrast to using longer distinctive codes or sequences each of 4096 chips to identify the BS and any of 16 users, for which matched filtering would be required for sequences of 4096 chips, this preamble construction enables the matched filtering to be applied to the much shorter sequences of 256 chips, with a consequent substantial reduction in computational complexity.
It has been found, however, that Golay sequences (and Gold code sequences which have previously been proposed) produce substantial cross correlation peaks for time shifts corresponding to 256 chip boundaries, corresponding for example to a communication distance of about 9 km. Such undesired cross correlation peaks are understood to be due to the repetitive nature of the constituent sequences in the signature patterns discussed above, and present a significant challenge if the delay due to communications distance can correspond to the preamble length or a multiple of the preamble length, and hence for situations where it is desired to use a relatively short preamble and relatively large cell sizes.
Accordingly, aspects of this invention seek to provide an improved preamble, and method for providing such a preamble, for use in an access channel of a communications system, a method of producing an extended sequence from a pair of constituent Golay sequences, and related apparatus.